A pine marten has been spotted in England recently, the first in more than 100 years.

The reemergence of Britain’s second-rarest mammal, a cat-sized relative of badgers and weasels, is a great story in itself.

But it may have another upside, as pine martens could be bad news for one of the UK’s least popular animals: the invasive grey squirrel.

Unlike pine martens, grey squirrels are not native to Britain. These North American ‘aliens’ were first introduced in the 1870s and soon made themselves at home. In the UK they are considered an invasive species – their ‘bark-stripping‘ harms the growth of new woodlands and has a big economic cost.

Grey squirrels’ success has also been to the detriment of the native red squirrel. Greys do not kill reds directly, but they do spread squirrel pox, a virus that causes distinctive ulcers on the reds’ eyes and nose, leading to death within a week. Grey squirrels themselves are unaffected – they’ve developed immunity.

Things are looking pretty dire for the UK’s red squirrels. Competition, disease and habitat loss mean that, if current grey squirrel control efforts were to stop, red squirrels would become extinct in Britain.

I’m interested in how pine martens fit into this struggle. Habitat loss, hunting for fur and predator control by game keepers meant they became practically extinct in England and Wales. However in Scotland and Ireland they are making a comeback – and where they are returning, grey squirrels are disappearing.

Why Ireland has red squirrels

The impact in Ireland has been particularly notable. A four-year study I published in 2014 found pine marten recovery in the Irish midlands was linked to such a significant decline in grey squirrel numbers that the once beleaguered red squirrel population was able to recolonise its former range, including woodlands which had been dominated by greys for more than 30 years.

The study provided the first evidence for what foresters and gamekeepers had been saying for years – where pine martens had returned to healthy numbers, grey squirrels had all but disappeared. But in areas with few or no pine martens, grey squirrels persisted at ‘invasive‘ levels.

Red squirrels on the other hand have coexisted with pine martens throughout much of Europe for tens of thousands of years. The two species evolved together.

While pine martens will very occasionally eat red squirrels, they don’t seem to have a negative impact on population numbers. In fact, in the Irish study, the areas that red squirrels had recolonised naturally were exclusively those with healthy pine marten populations.

Do pine martens eat grey squirrels?

We do know that more pine martens in an area means fewer grey squirrels, but we don’t yet know if this is down to direct predation. It does happen though: the first evidence of a pine marten preying on the American grey squirrel was also recorded in Ireland in 2013, and we are now looking for evidence of this in the Scottish borders too.

Grey squirrels are larger and less agile than red squirrels and typically spend more of their time on the ground, making them an easier prey.

However, having a healthy native predator around could also affect grey squirrels in various other ways: they might simply learn to avoid known pine marten areas, or they might spend less time on the ground foraging, leading to reduced fitness. Grey squirrels might even be suffering physiological effects such as stress-induced reproductive problems.

Ultimately we need to determine whether the pine marten could act as a natural biological control for the grey squirrel in Britain and Ireland. That’s why I’m now looking at Scotland, where there have been reports of grey squirrel declines after pine marten recolonisation since the early 2000s. I want to know if the two processes are linked.

Pine martens as pest eradicators?

There are several subtle but potentially important differences between pine marten populations in the two countries that I’ll need to take into account – Scottish pine martens can feast on field voles, for instance, a rich food source that isn’t found in Ireland.

Mass reintroduction of pine martens may be implausible but the creatures are moving south through Scotland and are literally just a few miles from the English border, so the process of natural recolonisation in England is almost underway.

The recent sightings in Shropshire may even mean the remnant Welsh population is spreading into England too. It is important to remember pine martens are very slow breeders however, and it will take the recovering population quite some time to reach levels healthy enough to potentially impact on grey squirrel populations.

Predators are a vital part of a healthy ecosystem and predator prey interactions have an important function. What’s happening in Ireland and potentially Britain with squirrels and pine martens is a great example of how restoring natural predators can reduce the damage caused by invasive species.

We are currently living in an unnaturally predator-poor environment, and it’s possible this has allowed some introduced species to reach ‘invasive’ levels, which has ultimately wreaked havoc on our ecosystem.

Interaction between pine martens and squirrels is fascinating from a scientific point of view and we still have lots to learn. But you don’t have to be an ecologist to appreciate the value of promoting one of Britain’s most beautiful native species in order to preserve another.

Emma Sheehy is Postdoctoral Research Fellow in Ecology at the University of Aberdeen.

This article was originally published on The Conversation. Read the original article.